The present invention generally relates to an aircraft cabin pressure control system and method and, more specifically, to systems and methods for controlling aircraft cabin pressure with an open-loop motor control system by applying a duty cycle offset to compensate for asymmetric motor loading.
For a given airspeed, an aircraft may consume less fuel at a higher altitude than it does at a lower altitude. In other words, an aircraft may be more efficient in flight at higher altitudes as compared to lower altitudes. Moreover, bad weather and turbulence can sometimes be avoided by flying above such weather or turbulence. Thus, because of these and other potential advantages, many aircraft are designed to fly at relatively high altitudes. The altitude to which an aircraft may fly is, in many instances, limited to a maximum certified altitude.
As the altitude of an aircraft increases, from its take-off altitude to its “top of climb” or “cruise” altitude, the ambient atmospheric pressure outside of the aircraft decreases. Thus, unless otherwise controlled, air could leak out of the aircraft cabin causing it to decompress to an undesirably low pressure at high altitudes. If the pressure in the aircraft cabin is too low, the aircraft passengers may suffer hypoxia, which is a deficiency of oxygen concentration in human tissue. The response to hypoxia may vary from person to person, but its effects generally include drowsiness, mental fatigue, headache, nausea, euphoria, and diminished mental capacity.
Aircraft cabin pressure is often referred to in terms of “cabin pressure altitude,” which refers to the normal atmospheric pressure existing at a certain altitude. Studies have shown that the symptoms of hypoxia may become noticeable when the cabin pressure altitude is above the equivalent of the atmospheric pressure one would experience outside at 8,000 feet. Thus, many aircraft are equipped with a cabin pressure control system (CPCS) which functions to, among other things, maintain the cabin pressure altitude to within a relatively comfortable range (e.g., at or below approximately 8,000 feet), allow gradual changes in the cabin pressure altitude to minimize passenger discomfort and maintain cabin-to-atmosphere differential pressure below nominal and maximum limits. Thus, many cabin pressure control systems control cabin altitude as a function of aircraft altitude, and do so in a manner and rate that will keep the cabin-to-atmosphere differential pressure less than the nominal limit.
Conventional cabin pressure control systems are designed to exhaust cabin air during flight in order to comfortably and safely pressurize the fuselage (cabin) so that high altitude aircraft flight can occur. Conventional CPCS design may utilize an electromechanically controlled outflow valve that is modulated to control the outflow of air from the cabin, thereby controlling cabin pressure. The electromechanically controlled outflow valve is comprised of an embedded software controller that spins a motor which drives a geartrain connected to a butterfly valve. In conventional CPCS designs, a brushed motor is spun in an open-loop drive scheme, that is, by applying a voltage directly across the motor terminals, without using speed feedback as a control parameter.
An open-loop drive scheme may lead to asymmetric motor response if the loading on the motor shaft is asymmetric, that is, the loading to the motor is only in one direction. In CPCS applications, the loading on the geartrain connected to the butterfly valve is unidirectional (in the closing direction) due to a mechanical spring and pneumatic effects, thus causing asymmetric loading on the motor. Asymmetric motor loading can cause undesirable CPCS response in an open-loop motor drive scheme, that is, without using motor speed feedback information to correct for the asymmetric loading. The undesirable CPCS response can be a source of dissatisfaction with aircraft operators.
Referring to FIG. 1, there is shown a control law logic 10 for a conventional CPCS according to the prior art. The input variable is the cabin rate error 12, which is the cabin rate command (not shown) minus the cabin rate actual (not shown). Generally, the term “cabin rate” refers to the change in cabin altitude relative to sea level, often expressed in sea level feet per minute (slfpm). The cabin rate command refers to a commanded rate of change in cabin altitude. Often, it is desirable to have a relatively small cabin rate command so that passengers in the aircraft cabin may not notice sudden changes in cabin pressure that may be associated with sudden changes in cabin altitude.
The cabin rate error 12 may be operated upon by a proportional-integral (PI) control scheme 14 as is known in the art. The output from the PI control scheme 14 may be a duty cycle command (proportional) 16 and a duty cycle command (integral) 18. The proportional duty cycle command 16, as is known in the art, may be summed with the integral duty cycle command 18 to provide an unlimited duty cycle command 20. The unlimited duty cycle command 20 may be controlled within preset saturation limits 22 to give a duty cycle command 24. This duty cycle command 24 may correspond to a certain voltage being applied directly to a motor to regulate a butterfly valve (not shown), as discussed above, in an open-loop system. However, as discussed above, depending on the pneumatic and mechanical loading in the system, the conventional duty cycle command 24 may be insufficient for proper regulation of the cabin altitude and cabin rate, thereby creating dissatisfaction with aircraft operators and passengers.
As can be seen, there is a need for an open-loop control logic and method that may compensate for asymmetric motor loading while appropriately regulating cabin air pressure.